Semiconductor memory devices are storage devices that can store data and can output the stored data, if necessary. Generally, the semiconductor memory devices may be classified into Random Access Memories (RAMs) and Read-Only Memories (ROMs). The ROMs are a non-volatile memory that retains their stored data even when the power supply is interrupted. Examples of the ROMs include PROMs (Programmable ROMs), EPROMs (Erasable PROMs), EEPROMs (Electrically EPROMs), flash memories, and the like. The flash memories are classified into NOR-type flash memories and NAND-type flash memories. Meanwhile, the RAMs are a volatile memory that loses their stored data when the power supply is interrupted. Examples of the RAMs include Dynamic RAMs (DRAMs) and Static RAMs (SRAMs).
Recently, semiconductor memory devices in which non-volatile materials are substituted for the capacitor of the DRAM have been introduced. Examples of the semiconductor memory devices include ferroelectric RAMs (FRAMs) employing ferroelectric capacitors, magnetic RAMs (MRAMs) employing tunneling magneto-resistive (TMR) films, and phase change memory devices using chalcogenide alloys. Especially, the phase change memory devices are non-volatile memory devices using phase change, that is, resistance change according to temperature change. The phase change memory devices have a relatively simple manufacturing process and can realize capacious memory at a low cost.
The phase change memory device includes a write driver circuit supplying a program current to a phase change material (GST) during a programming operation. The write driver circuit supplies the program current such as a set current or reset current to memory cells by using power supply voltage (for example, 2.5V or more) provided from an external apparatus. The set current changes the phase change material (GST) of the memory cell into a set state, while the reset current changes the phase change material (GST) of the memory cell into a reset state.